The present invention relates to electrophotography and more particularly, to a dry process developing method in electrophotography and a device employed therefor in which disadvantages inherent in the conventional dry process developing methods, for example, two component developing method employing non-magnetizable toner particles and magnetizable carrier material and one component developing method employing magnetizable toner particles are eliminated.
Generally, two component developing methods such as the cascade developing method, magnetic brush developing method, etc., are well known in the art and have been put into practical application. In such developing methods, electrically insulating non-magnetizable toner particles having average particle diameters of 10 to 15 .mu.m and particles commonly known as carriers are mixed for use. In the cascade developing method, the non-magnetizable tone particles are charged through rubbing against the electrically insulating carrier particles of bead-like shape to be attracted onto surfaces of the carrier particles and transported to a developing position of a developing apparatus, while in the magnetic brush developing method, the carrier is formed into magnetizable particles mainly of iron of approximately 75 .mu.m in diameter to be magnetically attracted in the form of magnetic brush bristles onto an outer cylinder or sleeve of a developing apparatus in a known manner. In the magnetic brush developing method, the non-magnetizable toner particles are charged through friction against the carrier to adhere to the surface of the carrier particles and transported to the developing position in the similar manner as in the cascade developing method mentioned earlier. The electrically conductive carrier particles also serve as a developing electrode positioned extremely close to a photosensitive member during developing.
The dry process two component developing method as described above, however, has various problems particularly related to the carrier in that such carrier only functions in charging and transporting the non-magnetizable toner particles or as the developing electrode (in the case of the magnetic brush developing method) without directly engaging in the developing itself. Thus it is not consumed during each copying, and gradually deteriorates with the increase in the number of copies taken, generally making it necessary to replace it after a predetermined number of copies have been taken. Although the life of the carrier seem to have been prolonged to a considerable extent due to the recent development of carriers having various coatings, replacement thereof is still required after use for a predetermined period of time. Furthermore, since the mixing ratio of the carrier to the non-magnetizable toner largely affects the quality of copied images, giving rise to adhesion of the carrier to the photosensitive member in some cases, stabilization of the mixing ratio, i.e., replenishing of the non-magnetizable toner at a constant rate is required. Although various improvements have conventionally been proposed for such stabilization of the mixing ratio to be put into practical application, the constant rate to replenishment is still difficult, with the developing apparatus tending to be undesirably large in size. Particularly, when the particle size of the carrier is too small or the mixing ratio is deviated to the carrier side, the carrier may adhere onto the surface of the photosensitive member in some cases, thus adversely affecting the quality of the copied images. Moreover, since the diameter of the carrier particle can not be made excessively small due to the above fact, the increase in the surface area of such carrier particle is inevitably limited, and depending on the mixing ratio of the carrier to the non-magnetizable toner, there are cases where uneven charging for the non-magnetizable toner may take place.
In order to overcome the disadvantages inherent in the two component developing method as described above, there has conventionally been proposed one component developing method employing magnetizable toner particles, and a direct type copying apparatus, i.e., copying appparatus which uses a photosensitive paper applied with photosensitive material, without effecting transfer, and which is based on the one component developing method has already been put into practical use. Meanwhile, although various attempts have also been made to apply the one component developing method to the copying apparatuses of transfer type, there are difficult problems to be solved related to physical properties in the developing and transfer in that conditions contrary to each other i.e., electrical conductivity during developing and electrical insulation during transfer are simultaneously required. More specifically, while the developing is successful in the case of the electrically conductive, magnetizable toner particles having high electrical conductivity, there is a disadvantage such that during electric field transfer onto plain copy paper, the polarity of the magnetizable toner is varied due to injection of charges thereto from the copy paper, thus resulting in the so-called phenomenon of "Blow-off" in which the toner once transferred onto the copy paper again leaves the same copy paper to cause non-uniform density and fogging in the copied images.
For eliminating the undesirable non-uniform density and fogging in the copied images as described above, there have conventionally been proposed various arrangements such as employment of electrically insulated copy paper (disclosed, for example, in Japanese Patent Publications Tokkaisho No. 50/90336, and Tokkosho No. 49/11576), pre-heating of copy paper (disclosed, for example, in Japanese Patent Publication Tokkaisho No. 50/43936), and uniform exposure of photosensitive surface to light before or during transfer (disclosed, for example, in Japanese Patent Publications Tokkaisho No. 51/26044 and Tokkaisho No. 51/96332), etc., none of which is, however, related to improvement of the one component developing method.
On the contrary, the electrically insulating magnetizable toner particles have problems related to developing. More specifically, since such electrically insulating magnetizable toner particles are not sufficiently stable in charging, the developed images tend to be undesirably soiled, and for eliminating such disadvantages, auxiliary means, for example, means for subjecting the electrically insulating magnetizable toner to corona charging within the developing apparatus is required as disclosed in Japanese Patent Publication Tokkaisho No. 50/117432, thus resulting in complication in the structure of the developing apparatus.
Although a developing method employing magnetizable toner having properties intermediate the electrically conductive magnetizable toner and the electrically insulating magnetizable toner has also been conventionally proposed, for example, in Japanese Patent Publication Tokkaisho No. 50/92137, it is quite doubtful whether such magnetizable toner can satisfactorily provide the properties of the electrically conductive magnetizable toner and electrically insulating magnetizable toner, while problems arise in such magnetizable toner from the viewpoints of difficulty in manufacturing thereof, stability under various temperatures and humidity conditions, etc.
Another disadvantage inherent in all one component magnetizable toner particles is that due to the necessity for employing magnetizable material, the cost of the toner tends to be high, and that since it is hard to increase the ratio of resin in such magnetizable toner particles, there is a difficulty in fixing thereof.
In order to eliminate disadvantages as described in the foregoing, the present inventors have completed novel developing method explained in detail later in the present application in which non-magnetizable toner particles and magnetizable toner particles are mixed to form developing material, while the magnetizable toner particles are adapted to function as a conventional carrier so as to be consumed in a similar manner to the non-magnetizable toner particles when attracted onto an electrostatic latent image formed on a recording medium or photosensitive member during developing.
In the novel method as described above, since the carrier (referred to as the magnetizable toner particles) is consumed, deterioration and the necessity for replacement of the carrier have been eliminated, but on the contrary, it has become necessary to replenish such carrier. For replenishing the toner and carrier, a developing material, a replenishing device having a replenishing vessel or tank in which the toner of non-magnetizable particles and the carrier of magnetizable particles pre-mixed at predetermined rate are contained for suitable replenishment may be conceived.
Additionally, there has further been proposed another known magnetic brush developing method, for example, in Japanese Patent Publication Tokkaisho No. 52/65443 wherein developing material composed of low resistant toner and high resistant toner, at least one of which is magnetizable toner, is employed for subjecting both of the toners to frictional charging, with electrostatic attraction therebetween being adapted to be larger than magnetic attraction of a magnet disposed in a sleeve so as to cause both of said toners to adhere to the electrostatic latent image for effecting the development. In the conventional method as described above in which the high resistant ordinary toner moves onto the photosensitive member, while attracting the low resistant magnetizable toner, it is only the high resistant ordinary toner that plays the main part in the developing. Accordingly, there is a possibility that the quality of copied images is varied depending on the amount of the high resistant ordinary toner in the developing material, thus adverse effects therefrom are to be expected if such rate is altered for some reason. Moreover, since the developing is mainly dependent on the electrostatic attraction due to the charge on the photosensitive member only exerting force on the high resistant ordinary toner, it is not considered to be possible to alter the amount of the high resistant ordinary toner, to a large extent, in order to obtain a predetermined level of the image quality.
Accordingly, an essential object of the present invention is to provide an improved dry process developing method for electrophotography in which mixture of electrically insulating non-magnetizable toner particles used for the conventional two component developing method and magnetizable toner particles used for the conventional one component developing method is employed as a developing material for eliminating disadvantages inherent in the known dry process two component developing methods and one component developing methods.
Another important object of the present invention is to provide an improved dry process developing method as described above which is applicable to electrophotographic copying apparatuses of various types for development in an efficient manner.
A further object of the present invention is to provide a developing apparatus equipped with a developing material supplying device for effecting the improved dry process developing method as described above under optimum conditions.
A still further object of the present invention is to provide a developing apparatus of the above described type which is simple in construction and stable in functioning, and can be readily incorporated into electrophotographic copying apparatuses of various types at low cost.